(NEXSTAR) — Mark Volman, a co-founding member of the 1960s rock-pop band The Turtles, died Friday, Sept. 5 at 78. Volman’s representative confirmed the news to multiple news outlets.

According to his rep, Volman died in Nashville after a “brief, unexpected illness.” Volman was diagnosed with Lewy body dementia back in 2020, though it’s unclear what role, if any, the disease played in his death.

The Turtles rose to fame in the 1960s, with a string of Top 40 hits, including a cover of Bob Dylan’s “It Ain’t Me Babe” and “She’d Rather Be With Me.” Though they produced eight albums across their career, The Turtles are perhaps best known for the mega-hit “Happy Together,” which went triple-platinum in the U.S. and reached no. 1 on the U.S. Billboard Hot 100 in 1967.

Volman was born and raised in Los Angeles, where he met fellow future “Turtle” Howard Kaylan in high school. The two formed a variety of proto-Turtles bands, including a surf rock outfit called The Nightriders, then The Crossfires, before settling on The Turtles. The group was more folk rock than their previous material.

By the 1970s, both Volman and Kaylan were dissatisfied with their recording label, White Whale Records, and its intended direction for the group. The Turtles began to wind down activity.

Volman and Kaylan later joined American music legend Frank Zappa’s band the Mothers of Invention, where they performed vocals until 1971. Throughout the ’70s and ’80s , Volman and Kaylan continued to work together, which included a radio show on L.A.’s KROQ-FM, soundtracking films, and recording back-up vocals and/or session performing for the likes of Bruce Springsteen, Alice Cooper and John Lennon.

Due to contractual stipulations with White Whale Records, Volman and Kaylan were unable to use their real names for performing after The Turtles, and the two (and other Turtles members) began touring as The Turtles featuring Flo & Eddie in 1983. Volman was nicknamed “Flo” and Kaylan was “Eddie.” The pair continued touring sporadically up until 2018, when Kaylan chose to stop touring. Volman continued touring up until his death.

Though their career was relatively short, The Turtles and “Happy Together,” live on across media. “Happy Together” has appeared in countless TV shows, films, and, if you’re a Millennial, a famous commercial for the Nintendo 64 game “Super Smash Bros.”

In 2023, Volman spoke to People about his Lewy body dementia, which causes protein deposits to develop in the nerve cells and brain, affecting thinking, memory and movement. Volman told People he even began hallucinating due to the deposits in his brain.

But the rocker appeared to be taking the challenges in stride, telling People: “I got hit by the knowledge that this was going to create a whole new part of my life. And I said, ‘OK, whatever’s going to happen will happen, but I’ll go as far as I can.'”

Volman is survived by partner Emily Volman, his daughters Hallie Volman and Sarina Miller, and his brother Phil Volman.